Conventionally, circuit boards have been used in electronic devices in which an electronic component, such as an insulated gate bipolar transistor (IGBT) of a power module or a switching module, is mounted. For example, Patent Literature (PTL) 1 discloses a circuit board in which a metal circuit sheet made of copper and formed into a circuit pattern is joined to a first principal surface of an insulating substrate, and a heat dissipating sheet made of copper and configured to dissipate heat generated from an electronic component mounted on the metal circuit sheet is joined to a second principal surface of the insulating substrate.
Generally, in a circuit board such as that described above, the thickness of the heat dissipating sheet is set to be substantially the same as, or slightly less than, the thickness of the metal circuit sheet. This is to balance the stresses generated on the first and second principal surfaces of the insulating substrate to reduce warpage.